In the past two weeks, we have witnessed both Sir Richard Branson on Virgin Galactic and Jeff Bezos on Blue Origin head up into space.
If you hear some speak, such as a founder of Amazon, Branson didn't go as high and did not reach the consensus height of where space starts.
Defining where space begins is a lot harder than it seems, though.
In theory, using the end of the atmosphere as the end of Earth and beginning of space would make sense.
But there are no boundaries with the atmosphere.
The atmosphere has different layers with different air densities, pressures, and temperatures - troposphere, stratosphere, mesophere, thermosphere, and exosphere.
Airplanes fly in the Troposphere, weather balloons in the Stratosphere, and satellites can orbit around the Earth in the thermosphere and exosphere.
However, there are no clear lines between these layers of the atmosphere.
You do not move 100 metres higher and it all of the sudden changes dramatically and you know you've entered a different layer.
Using gravity is also hard since gravity technically goes on forever.
Earth's gravity affects the Moon, Jupiter, the Andromeda galaxy, and even further.
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It is a tiny affect, just as the thermosphere has only a tiny amount of air, but it is there.
To solve this, Theodore von Karman attempted to define where space begins, by using a calculation where there is no longer enough atmosphere to create lift (which an airplane uses), and instead needs some sort of propulsion (what space capsules and satellites need) to stay up.
There is not one permanent height, though.
As the atmosphere changes, from density variations to even storms from the Sun, this height changes, from about 85 to 120km.
Since the definition can't change, Karman and the international community settled on 100km - the average in the range and a nice, round number that is easy to use and remember. That may sound arbitrary, and that is because it is arbitrary.
It doesn't affect how spacecraft are built - they will still need to be built to stay in orbit.
However, since the US does not like the metric system, parts of the US such as the military define it as 50 miles (80 kilometres), instead of 62 miles (100 km).
Again, like 100km, it is near this calculation from Karman and an arbitrary number that is easy to use and remember. Branson reached 85km and Bezos 107km.
Even though both launched from the US, Branson only reached space by imperial units and the US system, not metric units and the international community. Something Blue Origin and Jeff Bezos noted.
Perhaps a new way is needed - such as the height you need to make sure you can orbit around the Earth (as opposed to sub-orbital which means not doing a full circle).
Or perhaps is does not really matter, unless you are a billionaire trying to out-do your competition.
The real way to solve this problem is for the US to switch to the metric system.
However, getting to space is much easier than that.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU